Diatomaceous Earth (DE) is a natural substance derived from the fossilized remains of diatoms, which are tiny, single-celled aquatic organisms. These microscopic skeletons are primarily composed of silica. When ground into a fine powder, DE creates an effective, non-chemical method for pest control in gardening. Many gardeners are drawn to DE as a natural alternative to synthetic insecticides, but they must understand both its efficacy and the specific safety precautions required for its use.
Understanding Diatomaceous Earth Grades
Diatomaceous Earth is available in different forms, and the distinction between these grades is important for garden safety and efficacy. The powder’s composition is mostly silica, but the processing method determines its safety profile. Food Grade DE is the only type recommended for use in the garden or around people and pets. This grade is largely composed of amorphous silica, meaning it has not been heat-treated, and it contains less than 2% crystalline silica.
The alternative is Filter Grade (sometimes called Pool Grade) DE, which is chemically and structurally different. This grade is calcined, or heat-treated, to harden the particles for industrial filtration purposes. This heat treatment converts the amorphous silica into a high concentration of crystalline silica, which can be as high as 60%. Crystalline silica is a known health hazard if inhaled, and this grade must never be used for pest control or general gardening applications.
Safety Considerations for Humans and Pets
While Food Grade DE is considered non-toxic when ingested, the primary safety concern for humans and pets is inhalation of the fine dust. The particles can irritate the nose and nasal passages, and inhaling a large quantity may cause coughing or shortness of breath. Prolonged exposure to airborne DE, even the food-grade variety, can be problematic, especially for those with existing respiratory conditions like asthma. It is recommended to wear a dust mask or respirator when applying the powder to minimize airborne exposure.
For pets, the same inhalation risk applies, and the fine dust can also cause irritation if it gets into their eyes or on sensitive skin. When applying DE around the yard, keep pets away from the area until the dust has settled to prevent respiratory distress. Food Grade DE is considered safe for consumption in small amounts and is sometimes used as an anti-caking agent in animal feed.
Controlling Garden Pests and Protecting Beneficials
Diatomaceous Earth acts as a mechanical insecticide, meaning it kills pests through physical action rather than chemical toxicity. The microscopic particles have sharp, abrasive edges that damage the waxy, protective outer layer (cuticle) of an insect’s exoskeleton. Once this barrier is compromised, the absorbent DE powder draws out the insect’s internal moisture, leading to rapid dehydration and death. This physical mode of action prevents pests from developing resistance, a common problem with chemical pesticides.
DE is effective against a broad range of crawling and soft-bodied garden pests, including slugs, snails, aphids, earwigs, and cucumber beetles. However, the powder is non-selective, posing a threat to all insects that come into contact with it, including beneficial species like bees and ladybugs. To protect pollinators, gardeners should avoid dusting the flowers of plants where bees and butterflies actively feed. The best practice is to apply DE in the late evening when most pollinators are inactive, focusing the application on the base of the plant or the soil surface.
Effective Application Techniques
For DE to work effectively, it must remain dry, as moisture neutralizes its abrasive and absorbent qualities. Application should be timed for periods of dry weather. A light, uniform dusting is more effective than a thick layer, as pests will avoid heavy piles of the powder. Specialized dusters or shakers allow for a fine, even application, ensuring the powder adheres to the pest’s environment.
A common technique is to create an unbroken ring of DE around vulnerable plants to form a barrier against crawling pests. If an active infestation is present, a very light dusting can be applied directly to the foliage. Alternatively, DE can be mixed into a water slurry (about four tablespoons per gallon) and sprayed onto the plants, which is useful for reaching the undersides of leaves. Reapplication is necessary after any rain or heavy watering, as moisture renders the DE temporarily ineffective.